Spying three intruders inside their grassy sanctuary, a herd of horned bovines immediately went on the defensive. They grouped together and charged full speed across the field to within 10 feet, dropped their front shoulders and dared anyone to take another step.

"That's the yak in them," farmer Pete Mentzer explained.

The what?

"They're 50-50 yak and beef animal," he said of the creatures, which have long faces, low-set ears, thick coats and tails, and oddly angled horns.

Mentzer, who grew up farming in Loudoun County, and his partner, Jim Dumbrell, a retired British oil and gas pipeline consultant, are breeding yattle -- a cross between cows and yaks. "We laughingly call it Frankensteer," Dumbrell said of the crossbreed.

The odd pairings of farmer and oilman, cow and yak are indicative of the changing nature of farming in Loudoun, where operations that once raised corn, wheat and beef cattle across hundreds of acres have been sold off and broken up for development.

In their place have come nontraditional hobby farms -- organic vegetables, wine grapes, alpacas, llamas and now yattle -- often run by newcomers looking as much for a taste of country life as for the next big thing.

"After farms were sold off here, people had five or 10 or 20 acres, and they could not raise commodities profitably on those places," said Warren Howell, agricultural development officer with the county's Department of Economic Development. "They had to go into niche [products], and many of them have found that profitable."

Right now, yattle are the next big nothing. But Mentzer and Dumbrell hope they become the rage of a health-conscious society looking for a low-fat alternative to traditional beef.

Because yaks, with their horns and long coats, are cold-weather animals native to such places as Tibet, their fat is concentrated on the outside of their bodies, not riddled throughout, resulting in a low-cholesterol but slightly tough meat, Dumbrell said. By breeding the animals with beef cattle, the pair hopes to meld the best traits of both species.

A far cry from the days when the success or failure of the year's harvest governed family fortunes, ventures such as Dumbrell's are often pursued more out of whimsy than necessity.

"I get a lot of pleasure out of it," said Dumbrell, 65, who came to Loudoun 18 years ago and describes himself as a novice farmer. "I love working with animals."

Dumbrell said he got the idea to raise livestock after taking in a "seriously deformed" cow called Stumpy that was being shunned by the rest of the herd on a neighbor's farm. Seeking company for the creature, he explored his options.

"We were trying to find something that was different. We were thinking of alpacas, then bison, but the fencing for bison would be pretty expensive," given their strength and aggression. He summed up his reasons for choosing yaks with a single word and a grin: "Stupidity."

Realizing he'd need an actual farmer to help handle the up-to-1,300-pound animals, Dumbrell partnered with his neighbor, Mentzer, who, after 36 years of working for Pepco, decided he wanted to return to his roots and give farming another go.

Asked whether he ever thought he'd be partnered with a British yattle farmer, Mentzer, 69, said: "Absolutely not. No way, shape or form."

Dumbrell has 30 yaks on his 70-acre Purcellville farm -- which flies both British and American flags near its workshop -- while cattle and yattle are kept on neighboring farms where Mentzer works as a manager.

Unlike other crosses such as mules, which are infertile, only male yattle are sterile, allowing females -- which might or might not have horns -- to procreate with bulls.

Yattle are not the first example of two species being crossed to please the human palate.

In the early 1970s, beefalo -- crosses of bison and beef cattle -- was marketed commercially in the United States. That venture has largely flopped because of a low supply of animals, said Bob Weaber, a geneticist specializing in beef cattle with the University of Missouri.

American Beefalo International, an association of breeders primarily in the United States, reports registering 200 to 300 beefalo each year.

Weaber said crossing species is usually a risky venture given the fertility challenges involved. "You kinda gotta scratch your head as to why they'd do it," Weaber said of the yattle enterprise.

Michael Greger, director of public health and animal agriculture at the Humane Society of the United States, expressed little concern about the crossing of cow and yak, saying there could be benefits to the union.

"Some of these kinds of specialized animals may have higher welfare because the farmer may have put a lot of resources into these sort of niche species," Greger said. "If they have the only yattle in North America, [they're] going to call the vet, where as if a cow is down, they may let it languish or euthanize it."

Dumbrell and Mentzer's yattle operation is one of only a handful in the country, according to John Hooper, vice president of the International Yak Association. Hooper, who has a yak farm in Cold Spring, Minn., said his group estimates there are about 5,000 yaks in the United States, mostly in colder, higher-altitude climates, such as Colorado.

Hooper said he breeds some of his yaks with cattle but does not distinguish yattle meat from that of regular yaks when he sells it at local markets and restaurants.

"The meat is indistinguishable unless you do a scientific analysis of it," said Hooper, 57, who has 70 yaks and 15 yattle on his farm. "The flavor is the same."

Outside a pen of yaks on a recent afternoon, Dumbrell was decked out in a tweed hat, checkered shirt and Rolex watch ("All the farmers have them," he jokes of the pricey timepiece). Mentzer, sporting a Loudoun Cattlemen's Association hat, petted the animals -- lethargic in the Virginia heat -- through the fence.

"They don't like the weather," said Dumbrell, adding that yattle are capable of both mooing and grunting. "They're bilingual," he said.

So far, Dumbrell has bred eight yattle but has never tasted the meat. And although his financial future is not riding on it, he is fairly confident that the end product, still two to three years away, will be a hit.

"It's marketing," he said. "It could be the worst crap you've ever tasted in your life, but you'll pay for it, because it's taken eight years to achieve."